Voltage-gated potassium (Kv) channels repolarize excitable cells such as cardiac myocytes. Dysfunction of cardiac myocyte Kv channels causes life-threatening cardiac arrhythmias, but these channels are also useful antiarrhythmic drug targets. Thus, it is essential to understand their function, regulation and molecular composition, and determine how these differ regionally and between species. The current proposal draws from our preceding decade of work on cloning and defining the diverse physiological roles of members of the KCNE family of single- transmembrane-domain Kv channel ancillary subunits. Following our previous findings that KCNE2 mutations associate with inherited and acquired human ventricular arrhythmias, more recently we generated the kcne2 (-/-) mice line and used it to determine the primary roles of KCNE2 in adult murine ventricles - modulation of two Kv channels and their native current correlates: Kv4.2 (Ito,f) and, unexpectedly, Kv1.5 (IK,slow1). We also defined a new role for KCNE1, as an endocytic chaperone of the KCNQ1 a subunit, and found that both KCNE1 and KCNE2 can influence the a subunit composition of functional Kv channels. KCNQ1, KCNE1 and KCNE2 mutations associate with both atrial and ventricular arrhythmias. Kv1.5 mutations associate with atrial fibrillation (AF), and its function is relatively atrial-specific in human heart, potentially making it a useful target for atrial antiarrhythmics. Most forms of AF have no know genetic basis, and correlate with other factors such as aging, or following surgery to the heart or lungs. A fuller understanding of the native physiology of all these Kv subunits, and how they contribute to both inherited, and age-onset or post-surgery (acquired) forms of AF, is important to improving human cardiac health. Here, we propose to determine the roles of KCNE2 in atrial physiology and in the etiology of AF, utilizing kcne2 (-/-) mice (which exhibit pacing-induced AF), rabbit and swine models of post-operative AF, confirmatory experiments with human atrial tissue, and in silico multiscale atrial models. The studies comprise three Specific Aims. First, we will use a molecular approach to determine which atrial Kv complexes KCNE2 regulates, how its genetic disruption causes AF and Kv channel remodeling, how these mechanisms mirror post-operative AF in larger animals, and the role of Sp1, miR-1 and miR-133 in this remodeling. Second, we will use an electrophysiology/computer modeling approach to determine the function of KCNE2 in mouse and rabbit atria, compare the cellular functional effects arising from kcne2 genetic disruption and post-operative AF, and simulate the mechanistic basis for the resultant arrhythmias, from the cellular to the tissue level. Third, we will define the relationship between KCNE2, Kv1.5, and the intercalated discs (IDs), and determine why KCNE2 disruption prevents Kv1.5 ID targeting in the murine ventricles but not atria.